In details

The xylem


The conductive vessels of inorganic sap are formed by dead cells. Cell death is due to cell impregnation by lignin, a highly waterproofing aromatic compound.

The cell stops receiving nutrients and dies. The internal content of the cell is broken down, which becomes hollow and hard-walled since lignin also has the property of hardening the cell wall. Lignin deposition on the wall is not uniform. The cell, then hardened and hollow, serves as a conductive element.

There is also an interposed parenchyma (living tissue) that separates groups of conductive cells. These parenchymal cells are believed to secrete different types of substances that are likely to aid the preservation of dead xylem vessels.

There are two types of conductive cells in the xylem: tracheid and tracheal vessel element (either xylemic or woody).

  • Tracheids They are extremely thin cells, small in length (on average 4 mm) and small in diameter (on the order of 2 mm). When functional, tracheids are grouped into bundles and the ends of each other touch each other. At the end of each tracheid, as well as laterally, there are a series of punctuations or pores (tiny holes) that allow the passage of sap longitudinally and laterally.
  • Smaller than tracheids (on average 1 to 3 mm) but longer (up to 300 mm), vase elements They also have lateral punctuations that allow the sap to pass through. Its main feature is that at its ends the walls are perforated, that is, there is no fully insulating partition wall between one cell and another. The vessel formed by assembling various vessel elements is known as the trachea.
    The name trachea for the conductive vessel is derived from the similarity that vessel lignin ribs have to human tracheal cartilage and insect chitin ribs.

The conduction of inorganic sap

We have seen that roots absorb water from the soil through the absorbent hair region or the piliferous zone. From there, water travels through the cells of the cortex, endoderm and pericycle of the root. In endoderm the flow of water can be facilitated by the existence of so-called passage cells. Water reaches the xylem vessels and from these vessels reaches the leaf. In the leaf, it is either used in photosynthesis or released in perspiration.

The conduction of inorganic (or crude) sap is attributed to some mechanisms: root pressure, leaf suction, and capillarity.

  • Root pressure - The movement of water through the root is considered to result from an osmotic mechanism. Water in the soil enters the root hair cell, the concentration of which is higher than the soil solution. The root cell is less concentrated than the cortical cell. This, in turn, is less concentrated than the endodermal cell, and so on, until it reaches the xylem vessel, whose aqueous solution is most concentrated of all at this level. Thus, it is as if the water were osmotically pumped until it reaches the xylem vessels.
  • The suction exerted by the leaves - The currently accepted hypothesis for the displacement of xylem sap is based on the "suction" of water that the canopy exerts. This “suction” is related to the transpiration and photosynthesis processes that occur in the leaves. For this “aspiration” to be effective, two prerequisites are crucial: no air in the xylem vessels and a cohesive force between water molecules. The cohesion between the water molecules makes them stay together and withstand extraordinary forces, such as the very weight of the liquid column inside the vessels, which could cause them to separate. The existence of air in the xylem vessels would break this union and lead to the formation of bubbles that would prevent the rise of the woody sap. Woody vessel walls also attract water molecules and this adhesion, together with cohesion, are key factors in maintaining a new continuous column of water inside the vessel.
  • Sweating and Photosynthesis constantly remove water from the plant. This extraction generates a tension between the water molecules as the cohesion between them prevents them from separating. The vessel wall is also pulled due to the adhesion between it and the water molecules. In order to maintain the continuity of the liquid column, the replacement of water molecules removed from the canopy must be done by the root, which thus constantly supplies the xylem.
  • The effect of capillarity on sap conduction - Woody vessels are very thin, have capillary diameter. Thus, the rise of the xylem is partly due to capillarity. However, by this mechanism, water reaches heights well below 1 meter and, alone, this fact is insufficient to explain the rise of inorganic sap.